The present invention relates generally to a device and method for detecting material flaws using the eddy current principle, and more particularly to a flux focusing eddy current probe having separate excitation and pick-up coils, magnetically isolated from one another by a highly permeable flux focusing lens, for detecting different types of flaws or discontinuities at various depths within an electrically conductive material, and an automated method of monitoring crack growth rate and trajectory during fatigue testing. The automated method is not limitative and the invention can also be used for manual testing and fault isolation. The eddy current probe of this invention is simple in construction and operation while providing unambiguous signals of fault indication simplifying support equipment requirements, supporting self nulling capability in no fault conditions during surface fault detection negating the need for calibration standards, maintaining highly resistant lift-off characteristics, and providing highest signal levels at fault tips.
The use of eddy currents to inspect electrically conductive material for flaws is known in the art. Such methods are particularly useful in the nondestructive evaluation of conducting material for surface or internal faults and material thinning. An eddy current probe generally consists of a coil electrically connected to a current generator producing an alternating current within the coil. This generates a time-varying primary magnetic field which in turn induces current flow (eddy currents) in an electrically conductive material positioned in the vicinity of the coil. As described by Lenz's Law, the eddy currents, during their return path, create an opposing secondary magnetic field superimposed on the coil's primary magnetic field decreasing the coil's flux and modifying the coil's voltage effecting the impedance of the coil.
The eddy current's power and direction are dependent upon the specific impedance of the conductive material. In unflawed conditions, eddy current flow is generally parallel to the magnetizing coil's windings. Flaws, such as cracks, pits, and material thinning, in the conductive material create regions of higher resistances at the flaw location which affect eddy current flow. The direction change in eddy current flow reduces the opposing secondary magnetic field and consequently the voltage in the coil. An impedance detecting circuit, which may take the form of an inductive bridge, monitors coil voltage which, if different from an established no-fault condition, indicates a conductive material flaw. Sharp changes in impedance over a localized area would indicate the existence of cracks or other relatively small area flaws, whereas gradual changes in impedance over a broad region of the conductive material might indicate large-area flaws such as a grain change in the metal, an area of material creep, or material thinning. Though traditional eddy current probes use the same coil for magnetizing the conductive material and for detecting impedance variations caused by changes to eddy current flow, use of separate magnetizing and pick-up coils is known in the prior art. The same principle, however, of monitoring for variations in coil impedance as indicative of a conductive material flaw is applied. This conventional eddy current flaw detection technique often involves complex impedance planes necessitating special test electronics to achieve null balancing and known standards to calibrate probe responses to each type of flaw.
Multiple coils within a single probe can be used as separate magnetizing and sensing means, or they may be used in a more traditional fashion and independently operate the coils as bi-function magnetizing-sensing coils. In the case of a multiple coil probe, the coils can be juxtaposed in a matrix to provide a large detection area, or concentrically arranged to simultaneously detect flaws at various depths. Each coil of multiple coil probes is energized and monitored for impedance variations independently. Multiple coil probes use a shielding material high in magnetic permeability to provide a low reluctance path and divert potentially interfering magnetic fields to separate the coils from one another. The shielding is designed to isolate all interfering signals so that each coil's impedance can be independently balanced. This design provides an ability to perform material testing at several frequencies simultaneously. The conventional eddy current flaw detection techniques are employed which require special test equipment to analyze probe signals.
As the conventional eddy current probe separates from the test material, the eddy currents induced within the material rapidly decrease resulting in a similarly decreasing opposing magnetic field which directly affects the resistance and inductance of the probe's coil. Abrupt changes occur to the impedance balance being monitored for fault indications making traditional eddy current probes unusable during these lift-off conditions.
In addition to detecting existing flaws in conductive material, determination of fatigue crack growth criteria in structural materials is important to predict material fatigue failure limits. Current approaches to testing fatigue cracking include optical methods and other length measuring techniques such as crack mouth opening displacement gauge and four point potential drop method. Each of these procedures requires long periods of continuous monitoring by well trained operators to record fatigue crack tip locations and to adjust experimental controls in compliance with experimental designs. Though the crack mouth opening displacement gauge and four point potential drop method could potentially be automated to reduce operator time requirements, only overall crack length data can be provided and the fatiguing process must usually be stopped to make the crack length measurements.
Eddy current devices have also been used to monitor fatigue crack growth during fatigue. This approach to monitoring fatigue crack growth has been automated, though, again, only overall crack length data can be provided. The traditional eddy current probe which implements impedance measurement techniques to identify the presence of test material faults is not conducive to tracking the crack tip which supports crack trajectory as well as crack growth rate.